Fine conductive wires are used in the packaging of semiconductor devices to electrically couple a semiconductor die to a leadframe or a substrate. Normally, a connection is made between the die and the leadframe by ball-bonding the conductive wire to a bonding pad on the die, drawing the wire to a lead, and wedge-bonding onto the lead. These wires are generally on the order of 0.0254 mm to 0.0330 mm in diameter. Because of their fineness, wire sweep has been a constant problem in the plastic encapsulation of a semiconductor die. The high viscosity of the molding compound in its liquid state during the transfer molding process has the propensity to drag the wires along the flow path of the compound causing the wires to bend away from their original upright positions. Wire sweep is undesired because it poses a reliability risk to the functionality of the semiconductor device. Wires that are swept may come in contact with each other to cause shorting in the device. Wires may also be sufficiently swept so that the length of the wires touch the surface of the semiconductor die which would cause shortings between different components on the die. Therefore, it is always desirable to keep the wire sweep level to a minimum to protect the functional integrity of the semiconductor device. Ideally, the wires would remain rigidly fixed in place during the molding process, but this condition has not been possible prior to the present invention.
Plastic encapsulation of semiconductor dice has traditionally been accomplished through transfer molding. In the transfer molding process, the epoxy resin based compound is in the form of a solid pellet, which is then placed inside a pot in the mold die. The mold has been preheated to a set molding temperature. Molding is accomplished by a hot plunger coming into contact with the compound pellet. As heat is conducted through the pellet, the material plasticates and begins to flow. The viscosity of the plasticated compound is dependent on basic chemistry of the compound, the amount of filler present in the compound, and the temperature of the mold; that is, the higher the temperature, the lower the viscosity. However, molding compound only remains in a liquid state for a short period of time once it has been plasticated. Moreover, the viscosity will begin to increase again at a faster rate at higher temperatures than at lower temperatures. In effect, molding compound viscosity drops to a lower absolute value at higher temperatures, but stays there for a shorter period of time so that using higher molding temperatures results in having less time to perform the molding process. Because an epoxy resin based molding compound is a thermoset, it cannot be remelted and reshaped once the compound has gelled or hardened. Therefore, transfer molding must be completed before the compound gels. Thus, the time allowed to complete the transfer of compound into the mold die cavities is strictly limited to the time that the mold compound viscosity is such that flow is not hindered. It should be clear that the more viscous the compound is during the transfer process, the more drag it induces on objects, namely the wires, in its flow path. It is, therefore, tempting to transfer the compound as quickly as possible in molding. However, the disadvantage to using very fast transfer speeds is that the probability of inducing wire sweep increases with increasing transfer speed. The alternative is to use a lower molding temperature which delays the gel time of the compound thereby allowing a slower transfer speed to be used to help limit the amount of wire sweep. However, a slow transfer speed translates into a long molding cycle time which would affect production cycle time. Moreover, a slow transfer speed by itself is not sufficient to eliminate wire sweep. There are other important underlying factors, such as the viscosity of the particular molding compound, the length of the wires, the position of the wires relative to the flow path of the material.
Because of all of the above mentioned factors, wire sweep has always been unavoidable during molding, but controlling it is an established challenge in the production of semiconductor devices. Certain factors contribute to the overall difficulty in limiting the wire sweep. As stated previously, flowing molding compound exerts a drag force on the wires. If this force exceeds the strength of the wires or of the bonds, then the wires will bend in the direction of the force. Longer wires tend to sweep more easily than shorter wires; therefore, it is desirable to keep the wire lengths as short as possible. However, it is not always possible to keep the wire lengths short. Other constraints in the packaging technology are pushing the wires to longer lengths. It is often necessary to place a small die onto a large die pad or flag; that is, there is more than 0.64 mm clearance, a typical maximum constraint for this dimension, from an edge of the die to the corresponding edge of the flag. Having a die on a flag that exceeds the typical maximum allowable clearance often forces the connecting wires to be longer than desired. There is also a greater risk of sagging wires which would cause shorting in the device if the wires touch the metal flag. Furthermore, some packages are becoming larger in size in addition to having more pin counts. The QFP's (Quad Flat Packages) range in size from 7 mm.times.7 mm to 40 mm.times.40 mm. Larger package sizes generally correlate to longer wire lengths because some of the semiconductor dice that are placed within these high lead count packages are much smaller than the smallest flag size that can be designed into the leadframe.
Another factor that contributes to the difficulty of controlling wire sweep is the proximity of the wires to each other. The closer the wires are together, the more critical it becomes to limit the wire sweep to reduce the possibility of wires coming into contact with each other. Miniaturization of the geometry of circuit patterns on a semiconductor die is resulting in bonding pads being designed closer together. Moreover, die designers are putting more components on a single die to expand its functions. Increased functionality of each chip results in more I/O's. More output pads and smaller die circuit geometry combine to make the packaging process more difficult because the wires get longer and also placed closer together, both on the semiconductor die and on the leadframe. Increased pin counts forces the lead tips on the leadframe to be designed closer together. Some of the QFP's are already in production at 0.4 mm pitch between the leads and some are progressing toward 0.3 mm pitch and smaller.
An additional development in the packaging field that will contribute to the wire sweep problem is the emergence of fine pitched QFP (quad flat package) in molded carrier rings (MCR). The MCR poses a manufacturing problem because the molding process is more complicated than molding non-MCR packages. The MCR and the package can be filled sequentially with the MCR usually being filled first, or they can also be filled at the same time using a different gating configuration. The process window for this operation is very restrictive because the molding compound must be transferred quickly enough to fill both the MCR and the package before the compounds gels but it must also be transferred slowly enough not to cause excessive wire sweep. Because of the tight process window with this type of package, production yield can be affected since any deviation outside the established process window can cause molding rejects due to excessive wire sweep or incomplete filling of the part.
Several categories of transfer molds are currently in use for the plastic encapsulation of semiconductor dice. Mold tools can be of the conventional type, plate type, or automatic type. Conventional molds and plate molds utilize a single large pellet of molding compound to encapsulate multiple packages. A long runner system is required to accomplish this task. The mold die cavities are sequentially filled from the closest mold cavity to the pot and then continuing to fill subsequent cavities further away from the pot. Disadvantages in molding using these kinds of molds are long cycle times and insufficient process controls to limit the amount of wire sweep.
The emergence of the automatic multi-plunger molding system has mitigated the wire sweep problem. In this type of mold, the runners are kept very short, and each mold die cavity is usually fed by an individual pellet of molding compound. This scheme shortens the distance that the compound must flow which offers several advantages. Since the compound has a much shorter path of travel, most of the transfer can be completed while the compound is at its lowest viscosity state. Reduction in molding cycle time has also been achieved through the use of automatic multi-plunger molds. However, having automatic molds is not sufficient to eliminate wire sweep. For some of the older packages, such as the PDIP (Plastic Dual In-line Package) where the wires are short and the pitch between the leads is 2.54 mm, the use of automolding equipment is adequate in controlling wire sweep. However, it is still very difficult to control the wire sweep in the high pin count, fine pitch packages. Manufacturing productivity decreases because more molding related rejects are found with these newer packages because the automatic related rejects are found with these newer packages because the automatic molding systems cannot keep the wires rigidly fixed in place during the transfer of molding compound. Thus, new trends in die design and packaging pose a greater challenge to the wire sweep problem than the automold can solve.